The operating requirements for high pressure hydraulic hose such as an automotive brake hose are ever increasing, and accordingly improvements in the materials and structure of the hose are required. Current automotive brake hose members now typically comprise five layers--three of suitable elastomeric composition and two of suitable wound or braided fiber reinforcing layers. For example, the brake hose includes an extruded elastomeric core tube member and having an internal diameter of nominally 3.3 millimeters and an outside diameter of about 5.1 millimeters. The core tube is formed of a suitable elastomeric composition that is compatible with a specified brake fluid. Wound or braided on the outside diameter of this core tube is a reinforcing layer of suitable fibrous material. A function of the fibrous reinforcement is to prevent excessive expansion of the core tube when the brake system is under high pressure. Such fibers have been made of rayon, glass or polyvinyl alcohol. Extruded over the first braided or wound reinforcing layer is a cushion layer of a suitable elastomeric material. The cushion layer has been suitably formed of materials such as natural rubber, styrene butadiene copolymer rubber, polychloroprene and ethylene-propylene-diene monomer rubber (EPDM rubber). Wound or braided over the cushion layer is another fiber yarn reinforcing layer. Suitable fibrous materials for this second layer have included nylon, rayon, glass fibers or polyvinyl alcohol. Finally, an outer elastomeric layer is provided as a protective layer to prevent the ingress of water, oils and other contaminants from the environment in which the hose is expected to operate. Cover layers have been suitably formed of polychloroprene, ethylene propylene diene rubber, chlorosulfonated polyethylene and the like. The overall outside diameter of such five layer hose is about 10.5 to 11 millimeters.
Typically, this five layer hose is cut to a suitable length. Metal fittings are attached at each end of the hose and crimped to form a secure connection. The hose may then be connected in the brake system of the vehicle, usually between a steel tube and a hydraulic fitting at the brake member for the vehicle wheel. The hose thus provides a flexible connection between the relatively rigid portions of the brake system and the wheel brake. It contains the brake fluid that, when placed under pressure by the operator of the vehicle, actuates the brake system at the wheels of the vehicle.
Such a brake hose must meet several exacting standards. As stated, the inner core of the hose must be chemically compatible with the brake fluid and not undergo excessive volumetric expansion when pressure is applied to the fluid. The hose must be flexible and resist fatigue, cracking or other failure over a very wide range of temperatures. In this regard, such brake hoses are subjected to pressure tests, volumetric expansion tests, whip tests and high temperature impulse tests which measure its performance and resistance to failure despite continual flexing, subjection to high pressure impulses and exposure to high and low temperatures. The composite hose structure must prevent egress of the brake fluid and ingress of water to the brake fluid, which can adversely affect its properties, and the ingress of oil and other contaminants found in the vicinity of an operating brake hose.
While generally suitable hydraulic brake hose constructions have consisted of the use of EPDM elastomers incorporating carbon black and processing oils as each of the core layer, cushion layer and cover layer of the hose and polyvinyl alcohol fibers have been successfully used as the reinforcing materials in the two braid layers, it is desired to further improve the properties of these hoses, particularly against failure at repeated high pressure impulses from the brake fluid. EPDM rubber elastomers have found widespread application in brake hose because they seem to be impervious to water and prevent water from diluting and lowering the boiling point of the brake fluid. Similarly, braided polyvinyl alcohol fibers, especially when coated with a resorcinol-formaldehyde latex adhesive, have found to interact with and cooperate with the core tube, cushion and cover layers of EPDM elastomers to strengthen the hose construction against excessive volumetric expansion. However, while this combination of materials has otherwise provided excellent brake hose properties, it is weaker than other hose material constructions with respect to high temperature impulse tests.
Accordingly, it is an object of the subject invention to provide an improved high pressure brake hose structure employing EPDM elastomers for the respective elastomeric layers of the hose and polyvinyl alcohol fibers as the reinforcing layers of the hose. It is another object of the invention to provide such improvements in high temperature impulse characteristics of the hose without sacrificing other essential characteristics of the hose.